--- a/src/aircraft.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/aircraft.h	Wed May 02 09:39:11 2007 +0000
@@ -130,11 +130,11 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~Aircraft() {}
 
-	const char *GetTypeString() { return "aircraft"; }
+	const char *GetTypeString() const { return "aircraft"; }
 	void MarkDirty();
 	void UpdateDeltaXY(Direction direction);
-	ExpensesType GetExpenseType(bool income) { return income ? EXPENSES_AIRCRAFT_INC : EXPENSES_AIRCRAFT_RUN; }
-	WindowClass GetVehicleListWindowClass() { return WC_AIRCRAFT_LIST; }
+	ExpensesType GetExpenseType(bool income) const { return income ? EXPENSES_AIRCRAFT_INC : EXPENSES_AIRCRAFT_RUN; }
+	WindowClass GetVehicleListWindowClass() const { return WC_AIRCRAFT_LIST; }
 };
 
 #endif /* AIRCRAFT_H */